This invention relates to an image processing method, apparatus and medium, e.g. a method, apparatus and medium for subjecting input image data to density conversion processing.
One example of a typical image processing apparatus widely used in the prior art is an electrophotography-type apparatus using laser exposure.
In such an apparatus for forming images electrophotographically by laser exposure, excellent halftone grayscale expression is achieved by reproducing a grayscale image having a resolution of 200 dpi (dots per inch) in the main-scan direction of the laser beam and 400 dpi in the sub-scan direction (which is perpendicular to the main-scan direction) as the output image using an image signal having a resolution of 400 dpi in the main-scan direction and 400 dpi in the sub-scan direction, by way of example. In a case where an image that includes characters or lines is reproduced, the edges of characters or lines are reproduced in excellent fashion by making the resolution in the main-scan direction of the output image 400 dpi and the resolution in the sub-scan direction of the output image 400 dpi.
In certain image forming apparatus, images are reproduced at a resolution of 400 dpi in the main-scan direction and 400 dpi in the sub-scan direction in a copy mode wherein importance is placed on character reproduction, as when copying characters and maps, in comparison with the reproduction of images at a resolution of 200 dpi in the main-scan direction and 400 dpi in the sub-scan direction in an operating mode wherein emphasis is placed on halftoning, as when copying photographs that have been printed out or photographs appearing on photographic paper.
A copier has been proposed for dealing with a case where the image of a document contains characters and photographs. Such a copier subjects the edge of a character or fine line to area discrimination, performs image reproduction at 200 dpi in the main-scan direction and 400 dpi in the sub-scan direction in an area determined to be that of a grayscale image and performs image reproduction at 400 dpi in the main-scan direction and 400 dpi in the sub-scan direction in an area determined to be that of a character or fine line.
However, in view of the needs of users seeking images reproduced at higher image quality, it is expected that expression of the grayscale at 600 dpi will predominate in digital-type eletrophotographic printers.
If the space needed for one pixel is considered to be a square, one side of the square will have a length of 42.2 .mu.m for a resolution of 600 dpi in comparison with a length of 63.5 .mu.m for a resolution of 400 dpi. Accordingly, the specifications of the laser optical system and photosensitive drum should be optimized in terms of the diameter of the laser-beam spot and in terms of preventing the scattering of light on the photosensitive drum in order to deal with an increase in resolution to 600 dpi. However, redesigning the apparatus upon taking into account even the characteristics of the elements and materials used is not realistic when cost is considered. When, for this reason, resolution is changed from 400 dpi to 600 dpi using the currently existing basic arrangement of components for grayscale reproduction at 400 dpi, a problem which arises is that the amount of overlap between neighboring pixels becomes too large if exposure is performed by scanning with a laser beam diameter of 50 .mu.m in the main-scan direction and 65 .mu.m in the sub-scan direction, even though such a beam spot diameter is optimum for 400 dpi. This problem will be described with reference to FIGS. 1 and 2.
A laser-beam spot diameter 201 is designed to be 50.times.65 .mu.m so as to be optimized for a pixel 202 formed by a grid of 400 dpi. Under these conditions there is almost no overlap in the sub-scan direction (FIG. 1). However, if this optical system is employed as is and an image is formed by a pixel 203 composed of a grid of 600 dpi, an area in which neighboring pixels overlap is produced, as indicated by the shaded portion in FIG. 2.
If such overlapping portions exist, periodic pitch unevenness will appear in the output image in the sub-scan direction in the event that the traveling speed of the photosensitive drum in the sub-scan direction is not uniform, namely in a case where the precision at which the rotating speed of the drum is controlled declines in the system in which the photosensitive drum rotates.
Even when a digital latent image expressed by area modulation is formed by a plurality of isolated dots, the characteristic will approach that of an analog latent image if neighboring dots overlap in the manner described above. The end result is that it is difficult to realize optimum grayscale reproduction.